package com.sequencelogic;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;


// Digest class for file and message comparison
public class Digester {
   private static final char[] HexDigits = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };

   private static final String digestName = "MD5";
   private static MessageDigest digest;

   static {
      try {
         digest = MessageDigest.getInstance (digestName);
      }
      catch (NoSuchAlgorithmException e) {
         throw new RuntimeException ("Could not create an instance of " + digestName);
      }
   }

   public Digester() {
   }

   public static String digest (File file) throws IOException {
      digest.reset();
      return encodeHex (digest.digest (getBytesFromFile (file)));
   }

   public static String digest (String string) {
      digest.reset();
      return encodeHex (digest.digest (string.getBytes()));
   }

   // Encode the digest as a hex encoded string
   public static String encodeHex(byte[] data) {
      int length = data.length;
      char[] out = new char[length << 1];
      for (int i = 0, j = 0; i < length; i++) {
         out[j++] = HexDigits[(0xF0 & data[i]) >>> 4];
         out[j++] = HexDigits[0x0F & data[i]];
      }
      return new String (out);
   }

   private static byte[] getBytesFromFile (File file) throws IOException {
      if (file == null) {
         throw new IllegalArgumentException ("Digester: file cannot be NULL");
      }
      if (!file.exists() && !file.isFile()) {
         throw new IllegalArgumentException("Digester: file must exist and be a file");
      }

      InputStream is = new FileInputStream (file);

      long length = file.length();
      byte[] bytes = new byte[(int) length];

      int offset = 0;
      int numRead = 0;
      while (offset < bytes.length && (numRead = is.read(bytes, offset, bytes.length - offset)) >= 0) {
         offset += numRead;
      }

      if (offset < bytes.length) {
         throw new IOException("Digester: read failed " + file.getName());
      }

      is.close();
      return bytes;
   }

}